A senior Democratic senator has alerted federal investigators to a confidential letter she received regarding Brett Kavanaugh, Donald Trump’s conservative nominee for the US supreme court, in an extraordinary move that suggested she had been informed of possible wrongdoing.

Dianne Feinstein, who is the top Democrat on the Senate judiciary committee, said she had received information about Kavanaugh’s nomination from an individual who had strongly requested confidentiality. The letter was probably passed on to the FBI because the bureau is responsible for background checks into judicial nominees.

News of the letter came as Judge Kavanaugh faced fresh scrutiny about his relationship with another judge, who was forced to resign from the bench last year.

“I have received information from an individual concerning the nomination of Brett Kavanaugh to the supreme court,” Feinstein said in a statement.

“That individual strongly requested confidentiality, declined to come forward or press the matter further, and I have honored that decision. I have, however, referred the matter to federal investigative authorities,” she said.

A source who said they were briefed on the contents of the letter said it described an incident involving Kavanaugh and a woman that took place when both were 17 years old and at a party. According to the source, Kavanaugh and a male friend had locked her in a room against her will, making her feel threatened, but she was able to get out of the room. The Guardian has not verified the apparent claims in the letter. It is not yet clear who wrote it.

A spokesman for Feinstein declined to comment. The White House did not immediately return a request for comment.

While additional details about the letter were scarce, two media outlets have reported that the person who wrote the letter is being represented by an attorney, Debra Katz, who has been described in media reports as Washington’s #MeToo lawyer.

Katz has not responded to a request for comment from the Guardian. BuzzFeed reported that she was seen on Capitol Hill on Wednesday night, shortly after the Intercept first reported the existence of the letter.

As theories swirled in Washington about the contents of the Feinstein letter, Kavanaugh was facing fresh scrutiny about his relationship with Alex Kozinski, a powerful federal judge who was forced to resign from the bench after more than a dozen women accused him of sexual harassment and assault.

Kavanaugh worked as a law clerk for Kozinski in 1991 and maintained a relationship with the one-time chief judge of the US ninth circuit for years.

He testified under oath at Senate confirmation hearings last week that he was “shocked and disappointed” to learn about allegations of sexual misconduct against his former boss and friend. Kavanaugh said the news was like a “gut punch”.

But Kozinski’s alleged victims have said that his allegedly abusive behaviour, which included inappropriate touching and showing clerks pornography that he kept on his computer, was openly known in legal circles.

One victim who came forward as part of the #MeToo movement and asked not to be named in an interview with the Guardian, said the words “open secret” did not begin to describe the fact that Kozinski’s allegedly abusive behaviour was common knowledge in legal circles.

The person said she had no knowledge about what Kavanaugh might have known, but said: “It was wide out in the open. It is hard to overstate how sexualised and abusive it was, emotionally and psychologically.”

She added that “everyone knew Kozinski was a creep”, but that the behaviour was tolerated.

A judicial panel opened an investigation into Kozinski last year but then closed it after he voluntarily resigned, saying it lacked the authority to do more.

Among the allegations against the former judge are complaints that Kozinski circulated raunchy and offensive emails with clerks, friends and associates, in what was known as the Easy Rider Gag List. When news of the existence of the email distribution list first broke in the Los Angeles Times in 2008, it emerged that the inappropriate emails were sent periodically to friends, associates, law clerks and fellow judges on the federal bench, as well as attorneys and journalists.

At the time, Kozinski’s own lawyer defended the gag list, saying that it did not interfere with the judge’s ability to fairly decide cases.

Kavanaugh has said under oath that he could not recall whether he ever received any sexually inappropriate emails from Kozinski. He has not responded to several requests from senators that he refresh his memory and check his previous emails to see if he was on the gag list.

There is evidence that the gag list was widely distributed. In its 2008 article about the list, Paul Cappuccio, a Kozinski protege who worked as general counsel for Time Warner, told the Los Angeles Times he had been added to the list, and had assumed his name was on it because he was a former clerk.

Kavanaugh told senators in written responses that he contacted the disgraced judge after the allegations emerged last year, in order to check on the judge’s mental health. He also acknowledged receiving emails from Kozinski to his White House email address, indicating that the two kept in touch when he was working for the Bush administration.

Asked whether he ever referred clerks who wanted to work for the circuit judge, Kavanaugh said: “In my capacity as a law professor, it is possible that I talked to students who had applied or were interested in applying to clerk for Judge Kozinski, and assisted them.”

Kavanaugh is expected to be confirmed by the Senate in the coming weeks, despite a strong campaign of opposition by Democratic activists and progressive groups who say he represents a threat to reproductive rights and other progressive causes. Democratic senators are still seeking to stall his confirmation, however, by raising questions about his truthfulness under oath.

Kavanaugh is not the only high-profile official to have clerked for Kozinski. Alex Azar, who now serves as the Trump administration’s secretary of health, and who is a close personal friend of Kavanaugh, also worked as a clerk for the judge in 1991 for six weeks.

A spokeswoman for HHS told the Guardian: “The secretary had no knowledge of any circumstances that led to the later alleged sexual misconduct by Judge Kozinski, nor did he witness any sexually inappropriate behavior during his clerkship.”

At the time of his resignation, Kozinski said he had a “broad sense of humour” and candid nature when speaking to males and females. He apologised to clerks who had been made to feel uncomfortable, saying it had not been his intention. He also said he would not fight the allegations against him reported in the media, because it would harm his “beloved federal judiciary”.

The Senate judiciary committee announced on Thursday that it would hold a vote on Kavanaugh’s nomination on 20 September.

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